Fluid flow control regulators are used to provide a constant flow rate by means of a pressure differential regulating device that senses changes in upstream or downstream pressure and compensates for the change. Conventional regulating devices use an impeller that is sensitive to a variable incoming fluid pressure, P.sub.1, and a downstream fluid pressure, P.sub.3. The impeller reduces a valve opening when the differential pressure between P.sub.1 and P.sub.2 increases, and enlarges the valve opening, when the differential between P.sub.1 and P.sub.2 is reduced.
An impeller spring biases the impeller against P.sub.1. A lesser fluid pressure P.sub.2 acts on the opposite side of the impeller from P.sub.1. The impeller assumes a balanced condition when P.sub.2 plus the impeller spring force equals P.sub.1. This occurs when P.sub.1 and P.sub.2 have reached a stable condition.
If P.sub.1 increases, the impeller moves from its stable position because P.sub.1 is greater than P.sub.2 plus the spring force, thereby reducing the valve opening. P.sub.2 then increases until P.sub.2 plus the spring force again equal P.sub.1 at a new stable impeller position.
If P.sub.1 reduces, the impeller moves in the opposite direction from its stable position because P.sub.2 plus the spring force are greater than P.sub.1. The impeller moves until P.sub.2 reduces to a level where P.sub.2 plus the spring force equal P.sub.1 at a new impeller position corresponding to a valve position that restores the desired flow rate. The impeller thereby automatically adjusts to restore a predetermined pressure difference between P.sub.1 and P.sub.2 which is determined by the force characteristic of the spring.
The impeller is also indirectly sensitive to the difference between P.sub.2 and P.sub.3 (the downstream pressure). The impeller does not directly sense P.sub.3. However, the required area of the outlet valve opening must increase or decrease based on the differential pressure between P.sub.2 and P.sub.3. As P.sub.3 increases, the efficiency of the valve hole decreases and thereby reduces the flow which increases P.sub.2 affecting the pressure balance on the impeller which then moves in the opposite direction from its stable position because P.sub.2 plus the spring force exceeds P.sub.1. The flow accuracy of the regulating device is not significantly affected by P.sub.3, however when the difference between P.sub.2 and P.sub.3 becomes very large, for example, 12,000 p.s.i., then the impeller must travel a greater distance, increasing the compression of the spring, which changes the pressure differential and creates an error in the regulating device.
Another problem with conventional flow regulating devices used to accommodate a high pressure but low flow rate condition is the capacity of the sensing orifice device through which the incoming fluid passes as it flows from the high pressure side (P.sub.1) to the low pressure side (P.sub.2) of the impeller. The orifice device can be adjusted to increase or reduce the desired flow rate.
The orifice hole size is critical. A high pressure, low flow situation requires a very small hole. However, a small hole size tends to become plugged with material carried by the fluid. Therefore it is desirable to have a hole with the largest area possible. An example is where the flow rate must be maintained between a fraction of a gallon and 1250 gallons per day, with incoming pressures as high as 12,900 p.s.i.g.
Hole size can be increased by providing a resistance to flow through the device by means other than reducing the hole size, such as by using a capillary device.
Capillary devices have been used in other types of technology, by using a threaded member inside a tube which may be either smooth or internally threaded to form a very small, but long, capillary path. See for example: U.S. Pat. Nos. 2,265,888, issued Dec. 9, 1941, to Rudolf Beck for "Liquid Level Indicator"; 2,568,123, issued Sep. 18, 1951, to Herman M. Goldberg for "Pressure Reducing Device for Refrigerating Apparatus"; 3,841,354, issued Oct. 15, 1974, to Roy Edward McDonnel for "Flow Regulating Device"; 3,791,619, issued Feb. 12, 1974, to Alfred W. Pett for "Valve Construction"; 3,143,145, issued Aug. 4, 1964, to James M. Kauss for "Method and Means of Controlling the Rate of Fluid Flow" and 2,850,038, issued Sep. 2, 1958, to Hubert A. Shabaker for "Flow Control Device"; and Norwegian Patent No. 923962.
As far as we are aware, no such capillary device has been used to adjust the flow rate in a differential pressure operated flow control device.